US President Donald Trump has criticised his country's visa system in the wake of a terrorist attack in New York City that left eight dead, blaming Democrats and pressing for a "merit based" program for immigrants to the United States.

"The terrorist came into our country through what is called the Diversity Visa Lottery Program, a Chuck Schumer beauty. I want merit based," Mr Trump tweeted, referring to the Senate Democratic leader.

"We are fighting hard for merit based immigration, no more Democrat lottery systems. We must get MUCH tougher (and smarter)," he also tweeted. Senator Schumer hit back, saying in a statement he had "always believed and continue to believe that immigration is good for America".

"[Mr Trump is] politicising and dividing America, which he always seems to do at times of national tragedy." Since taking office, Mr Trump has been pressing for a ban on travellers entering the United States from some predominantly Muslim countries.

"I have just ordered Homeland Security to step up our already extreme vetting program. Being politically correct is fine, but not for this!" he also wrote on Twitter.

Six victims were pronounced dead at the scene and two more at a nearby hospital, Police Commissioner James O'Neill said.

Five of the dead were Argentinian tourists, visiting New York as part of a group of friends celebrating the 30th anniversary of their high school graduation, the Government there said. Belgium's Foreign Minister said a Belgian citizen was also among those killed.

A US law enforcement official described the suspect as a US immigrant born in Uzbekistan, a landlocked, predominantly Muslim country in central Asia that was once part of the former Soviet Union.

CNN and NBC News reported that the man, identified by officials as Sayfullo Saipov, entered the US in 2010. Uzbekistan's President Shavkat Mirziyoyev told Mr Trump his Government would do all it could to help investigate the "extremely brutal" attack.

Although authorities from the mayor's office to the US Department of Homeland Security all swiftly branded the attack an act of terrorism, Mr Cuomo stressed that the suspect was believed to have acted alone.

Asked whether the suspect had been known to authorities before the attack, Mr Cuomo said in a CNN interview, "it's too early to give you a definitive answer".